Hydrate plugs are sometimes formed in oil wells and pipelines transporting oil and gas. The plugs are apt to form in pipes where the pressure is high and the temperature low. This may in particular occur in offshore wells.
In order for hydrate plugs to form in wells, the following conditions must be present:                Access to free water (free water means water in liquid form as a separate phase or dispersed in the hydrocarbon phase).        Access to light gas molecules (C1, C2, C3, iC4, CO2, N2, H2S).        Relatively high pressure.        Relatively low temperature.        
Hydrates are mixtures of water (as ice) and methane gas. The methane gas occurs in cavities in the ice and changes the physical properties of the ice. The presence of methane will, inter alia, lower the melting point, but the most important effect is the release of gaseous methane when the hydrate is melting. The melting of the ice will lower the volume, but the released methane gas will increase the pressure (1 m3 of ice can release up to 180 Sm3 of gas).
Several methods exist for inhibiting the formation of hydrate plugs, but nevertheless hydrate plugs sometimes form, as mentioned above. Hydrate can cause problems in wells, process systems and transportation pipelines. Massive hydrates which close the flow cross section can cause serious operating problems.
Small amounts of hydrate formation can put valves out of function or hinder well operations. These problems can have serious safety and economic consequences.
Hydrate plugs can be expected to form in many types of operations, such as cable operations, coiled tubing, hydraulic pipeline pressure operations, pump operations, leak testing, pumping of well fluids, input and/or output of equipment/tools, shut down of flow lines/gas lift lines, perforation of tubing, flow operations, well cleaning and change of christmas trees.
The standard method for removing a hydrate plug is to inject Methanol (MeOH), mono ethylene glycol (MEG), triethylene glycol (TEG) or brine (KCl, NaCl, CaCl2) and maintain relative high pressure at the top of the well. When injecting the hydrate inhibitor, it is important to note that it may be difficult (time consuming, days, weeks or even months) to get the inhibitor down to the hydrate plug, due to the long distance from the top of the well to where the hydrate plug is located.
In order to increase the efficiency of the chemicals and to reduce the fluid requirements, the chemicals may be delivered directly at the plug through coil tubing. However, it normally takes a long time to get coil tubing equipment mobilized and heavy coil tubing equipment must be lifted as “Heavy Lift” onto the rig. This means that critical weather limitations exist for heavy lift to be performed on platforms, especially onto semi-submersible rigs and Tension Leg Platforms (TLP). In addition to this, considerable time is needed to rig up the coil tubing equipment on the rig. A relatively large crew is also needed to operate the coil tubing equipment.
Another method is to drill through the plug by using coil tubing. But again, it normally also takes a long time to mobilize the coil tubing equipment and, again, the heavy coil tubing equipment is susceptible to the critical weather limitations for heavy lift onto similar platforms like semi-submersible rigs and (TLP) Platforms. Considerable time is also needed here to rig up the coil tubing equipment. A relatively large crew is also needed to operate the coil tubing equipment.
From U.S. Pat. Nos. 5,619,611 and 6,343,652 is known a method for unplugging pipes by lowering an electric heat device down to the plug. The heat device is mounted inside an encapsulation with a blunt end face. The heated end face will rest against the plug and melt it. Due to the small contact area between the tool and the plug, the heat transfer will be slow. The use of wire line tractors to transport the tool in deviated wells is also described. However, the tractors described are well known in the art but are too small to provide any appreciable force between the plug and the tool. There is also the danger of accidental release of the hydrate plug upwards due to high pressure from below. As far as we know, this method is currently not in commercial use.